Fuel cells produce electricity from chemical reactions. The chemical reactions typically cause a fuel, such as hydrogen, to react with air/oxygen as reactants to produce water vapor as a primary by-product. The hydrogen can be provided directly, in the form of hydrogen gas or liquid, or can be produced from other materials, such as hydrocarbon liquids or gases. Fuel cell assemblies may include one or more fuel cells in a fuel cell housing that is coupled with a fuel canister containing the hydrogen and/or hydrocarbons. Fuel cell housings that are portable coupled with fuel canisters that are portable, replaceable, and/or refillable, compete with batteries as a preferred electricity source to power a wide array of portable consumer electronics products, such as cell phones and personal digital assistants. The competitiveness of these fuel cell assemblies when compared to batteries depends on a number of factors, including their size, efficiency, and reliability.
In a high temperature fuel cell system, such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a reducing flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the reducing flow typically comprises a mixture of a hydrogen-rich gas created by reforming a hydrocarbon fuel source and an oxygen source, such as air, water vapor or carbon dioxide. The fuel cell, typically operating at a temperature between 500° C. and 1000° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ions combine with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The electrons from the negatively charged ions are routed back to the cathode side of the fuel cell through an electrical circuit completed between the anode and the cathode, resulting in an electrical current flow through the circuit.
Challenges exist in portable fuel cell systems which are exposed to a variety of operating environments. Prior art has discussed sensing oxygen in the ambient air and ceasing system operation when a predetermined threshold is met. However, this results in a system which is frequently not providing power to the user, and therefore unreliable.